There's been signs for a while. The International Advertising Bureau set HTML5 as the new industry standard in interactive marketing. Mozilla created *shumway' to shun flash away. Even Amazon banned flash ads on their network. Now, announced last week, Google are blocking flash ads in Google chrome. Chrome users will now actively have to choose to animate advertising content, if they find it interesting. The official reason is because flash "increases page-loading times and kills battery more quickly", but we all know that Flash ads are also how banner hijackings happen, so it's a security issue too.
Adobe has been trying to move people on to Adobe Air for some time, but HTML5 has been the easy use option that gained popularity quicker. Advertisers can still run flash banners on Googles ad network, and hope people select the option to play them, but with most browsers rejecting flash by default now, it's high time to stop using flash in banners.